1. Field of the Invention
The present invention relates to a hydraulic brake system and particularly to such a hydraulic brake system in which a hydraulic pressure in a wheel cylinder is increased and decreased by controlling a linear solenoid valve.
2. Discussion of Related Art
For example, Japanese Patent Application Publication No. 2003-252189 discloses a linear solenoid valve including a housing: a poppet valve provided in the housing; a spring that biases the poppet valve in a direction to close the valve; a solenoid that drives, upon supplying thereto of electric power, the poppet valve in a direction to open the valve. This linear solenoid valve is employed by, e.g., a hydraulic-pressure control device of a hydraulic brake system for use in a vehicle, and is used for controlling a hydraulic pressure in a wheel cylinder. In a hydraulic brake system including a linear solenoid valve, a degree of opening of the linear solenoid valve can be adjusted by controlling an electric power supplied to a solenoid of the valve. Therefore, a hydraulic pressure in a wheel cylinder of the hydraulic brake system can be easily controlled.
However, when the hydraulic pressure of the wheel cylinder is increased and decreased in a state in which the linear solenoid valve is opened, self-excited vibration, i.e., a phenomenon that a valve member of the valve continuously vibrates may occur. The more the amount of brake liquid that passes through the linear solenoid valve is, the more likely the self-excited vibration is to occur. The above-indicated Japanese Patent Application Publication proposes to restrain the occurrence of self-excited vibration by pre-charging, before a braking operation is started, the brake liquid into the wheel cylinder and thereby reducing the amount of brake liquid that passes through the linear solenoid valve.
Meanwhile, the self-excited vibration tends to occur more likely if the hydraulic pressure in the housing of the linear solenoid valve is excessively decreased. In particular, in the case where a pressure-decrease linear solenoid valve is communicated with a suction port of a pump via a liquid passage, a hydraulic pressure in a housing of the linear solenoid valve may be excessively decreased when the pump is operated, which leads to causing the self-excited vibration to occur more likely. In addition, it is speculated that if the air dissolved in the brake liquid in the housing separates from the brake liquid to produce air bubbles, the self-excited vibration occurs still more likely. However, the technique disclosed by the above-indicated Japanese Patent Application Publication cannot restrain the self-excited vibration caused for those reasons.
Thus, the conventional hydraulic brake system can be improved with respect to the art of increasing its utility, e.g., the art of restraining the occurrence of self-excited vibration.